evp: Use functions instead of direct structure field references

evp: Use functions instead of direct structure field references

AES and chacha20poly1305 also have some codes that directly reference
the fields in the EVP_CIPHER_CTX structure, such as 'ctx->buf' and
'ctx->encrypt', in order to make the code style uniform, use the
corresponding interface API instead of direct field references.

Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16465)